It has been well known that automobiles and various industrial machines and equipments are provided with a variety of sensors to detect targets of detection such as, for example, the number of revolutions, temperatures and vibrations so that those detected parameters can be used in controlling and state observation of those automobiles and industrial machines and equipments. Outputs from the sensors are generally transmitted through closed circuits and wires, but experienced difficulty often arises in locating places of installation of those closed circuits and wires. Under those circumstances, the wireless sensor system is currently utilized, in which detection signals are transmitted by the utilization of electromagnetic waves. Transmitters used therein are generally provided with a compact battery.
Also, in the ABS (Anti-lock Brake System), in which the number of revolution of a vehicle wheel is detected by a revolution sensor so that the automobile braking can be controlled, the wireless system has been suggested, in which in order to avoid a possible accident which may be brought about by damage to the sensor wiring within a tire housing and also to minimize the cost of assemblage, the use of harnesses extending between the vehicle wheel and the vehicle body structure is eliminated and, instead, as a revolution sensor, the detection signal thereof are transmitted in the form of electromagnetic waves. (See the Japanese Laid-open Patent Publication No. 2001-151090.) In an exemplary revolution sensor system of this kind, a multipolar rotary electric generator is used to accomplish the supply of an electric power for the sensors and an electric power for the transmitters as well as to accomplish the detection of the number of revolutions by means of a self-generation of the electric power, so that it can be assembled compact in size with no need to supply the electric power from the vehicle body structure to the revolution sensors. (See the Japanese Laid-open Patent Publication No. 2002-55113.)
Also, the Japanese Laid-open Patent Publication No. 2003-146196 discloses the use of a self-diagnosing circuit in a revolution sensor incorporated wheel support bearing assembly having a wirelessly transmittable revolution sensor used therein, in which the supply of an electric power to the sensors and a radio wave transmitters is carried out by an electric generator that concurrently serves as the revolution sensor, but also has made mention of the wireless supply of an electric power from an outside power source.
The Japanese Laid-open Patent Publication No. 2003-58976 discloses the transmission of the sensor signals in the form as digitalized. As an electric power source, a battery or an electric generator is used therein.
In the wireless sensor system utilizing the battery as an electric power source as discussed above, the battery is limited to a specific lifetime and, accordingly, replacement of the battery is required when the electric power stored therein is consumed, making it complicated to maintain the lifetime of the battery. It also involves in an environmental problem associated with disposal of the battery.
In the revolution sensor system utilizing the self-generated electric power as discussed above, since generation of the electric power takes place only when and after the vehicle wheel rotates, detection would be instable at an extremely low speed approximating to a halt although it would work stably at a speed of about 10 Km/h falling within the operating range of the ABS. Also, it cannot be applied to detection of a target of detection other than revolution, for example, temperature or the like.
Also, in the wireless electric power supply system, a large electric power is required to be transmitted for electric power supply since the efficiency is low as compared with that through the closed circuit and wires and the generation of the electric power by means of the electric power generator. However, to continuously transmit the large electric power in view of failure of the supply of the electric power brings about a problem that the electric power consumption of the system as a whole tends to increase.
As discussed above, the wireless sensor system requires a stable securement of the electric power.
On the other hand, in the wireless sensor system, the sensor signal to be transmitted by wireless is susceptible to influence brought about by disturbance noises and insufficiency is encountered with in terms of the system reliability. By way of example, in the system disclosed in the Japanese Laid-open Patent Publication No. 2003-146196 referred to above, since the supply of the electric power by wireless is carried out, the electric power can be available at all times, but there is a problem in that the sensor signal is susceptible to influence brought about by the disturbance. Also, where a plurality of wireless sensor units are employed, not only is it necessary to change the frequency of transmission of the sensor signal for each of the wireless sensor units, but also a plurality of signal receiving circuits each for the respective particular sensor signal transmission frequency are required. The electromagnetic waves used for the wireless supply of the electric power can be easily rendered to be non-modulated electromagnetic waves. However, in such case, in order to avoid interference between the electromagnetic waves for the electric power supply and the electromagnetic waves for the transmission of the sensor signals, a different frequency must be used for each of the electromagnetic waves for the electric power supply and the electromagnetic waves for the transmission of the sensor signals, or a different plane of polarization must be used for each of the electromagnetic waves for the electric power supply and the electromagnetic waves for the transmission of the sensor signals.
In the system disclosed in the Japanese Laid-open Patent Publication No. 2003-58976 referred to above, since the sensor signal is digitalized before it is transmitted, the sensor signal is robust against the influence brought about by the disturbance, but it involves problems similar to those discussed above since a battery or an electric power generator is used as the electric power source.
As discussed above, numerous problems are encountered in the wireless sensor system in connection with securement of the electric power source and reliability of the sensor signals.